Investors bought 15 downtown buildings for a total of $2.7 billion, up from 11 properties that traded for $2.1 billion in 2010, according to Jones Lang LaSalle Inc., the Chicago-based real estate firm. But the total still was about half the sales volume reached before the real estate market crashed in 2008.

“To finish the year at $2.7 billion is, in the grand scheme of things, an above-average year,” says Bruce Miller, managing director at Jones Lang and head of the firm’s office investment sales team here. “A lot of us still feel like it’s down because we remember the market peaks of 2006 and 2007, but overall last year was a strong year.”

Over the past 15 years, downtown Chicago has averaged about $2.1 billion in sales annually, making 2011 relatively strong historically, Mr. Miller says. With several other office towers on the market and lending loosening up, many real estate observers believe sales will continue on a similar pace this year.

Less than a month into 2012, Shorenstein Properties LLC already has closed on its $228 million acquisition of the former Apparel Center, 350 N. Orleans St.

Last year’s 15 office tower sales totaled more than 11.1 million square feet, with an average price of $247 per square foot. Leading the list of biggest deals was UBS Realty’s $400.3 million acquisition of the Leo Burnett Building at 35 W. Wacker Drive and CommonWealth REIT’s $390 million purchase of 600 W. Chicago Ave.

“You’ll see buyers out there,” says Stephen Quazzo, CEO of Chicago-based Pearlmark Real Estate Partners LLC, which sold the 200 W. Madison St. building it co-owned with Tishman Speyer Properties L.P. for $217.5 million in September. “You’re starting to see rents improve, and for core assets, there’s financing readily available.”

Yet the market took a pause near the end of the summer, as the debt-ceiling debate and European financial crisis spooked many investors. And one of the biggest expected deals of 2011 never happened: the sale of Willis Tower, 233 S. Wacker Drive. The 1,450-foot-high skyscraper has been taken off the market.

Until recently, investors were mainly interested in two types of buildings: the most-expensive, fully leased towers — so-called core properties — and properties that had fallen into to loan trouble. But now demand is growing for properties in the middle: office buildings that have some vacancy but aren’t considered distressed, Mr. Miller says. Five downtown buildings with occupancy between 60 percent and 86 percent changed hands last year.

“It’s a sign of a progression of a market when you start to see investors coming in from both ends of the spectrum,” he says. “When you see buildings with 75 percent occupancy trading, that is a sign of an improving market. Buyers are taking a little more risk without necessarily looking at a distressed situation.”

Downtown sales still could be slowed by a variety of economic factors, especially if employers remain cautious about hiring. But interest rates are low and debt and equity are more readily available than in recent years, Mr. Miller says.

“Appetite by investors and appetite by lenders is high,” Mr. Miller says. “That typically translates into good sales totals.”